Ok, my opinion,



I would trust the technology in model airplanes more than the big V8s from Chevys and Mopars. Those $3K little radial engines are cool.

My 3.8 conversion the block was already MLS prepped but I used early DOHC heads as they were. I can not say they are sealed perfectly but they work fine. Same thing with a couple of 4G63s that I freshened used 200K engines and am running MLS where they were not originally speced. I found on these they wanted more torque on the head bolts to stay sealed.
Give the surfaces of the little Zuki a good look, if they are shiny the MLS gasket will seal perfectly as delivered. If they have a machined surface they may weep lightly but will not be an issue other than needing the coolant topped up periodically.
I expect when the head was serviced it got surfaced, what finish did they use and had you specified the use of an MLS?

Where it is common to block sand the top of an engine block, I would finish that up with a finer paper such as 400 grit. Obviously care and technique are required to block sand it but it is far better than scotch pads in an air motor.
I would expect the top of the Zuki block should have been quite shiny & smooth originally.

I then would go ahead and use the MLS as delivered. If you want or need to use any sealants just do so around the outer perimeter and not near the bores themselves.